When radiation-curable inks and varnishes are applied to the (non-contact) surface of primary or secondary packaging intended for foodstuffs, then any contamination from the package impacting the foodstuff should fall within the guidelines set out by Article 3 of Regulation (EC) No 1935/20041, as recommended by EUPIA (European Printing Ink Association), requiring that materials and articles in contact with food to be “manufactured in accordance with good manufacturing practices, so that under normal or foreseeable conditions of use, they do not transfer their constituents to food in quantities which could: endanger human health; or bring about an unacceptable change in the composition of the food; or bring about a deterioration in the organoleptic characteristics thereof”.
EUPIA has recommended that Article 3 of this provision be followed when producing printed matter for food packaging and has produced a detailed guideline for the selection of raw materials intended for printing inks for food packaging, along with guidelines on the testing of printed matter to ensure that regulatory requirements are achieved. In particular, it is a requirement of the inks of this invention, that when they are cured under the action of UV light that the level of contaminants arising from the cured ink film and reaching the foodstuff should fall below the specific migration limit for any material, in the instances where specific migration limits (SML) exist for those compounds. Where no SML exists for a specific component then the following migration limits apply: “A target migration limit of no concern for non-evaluated substances of 10 ppb is the ultimate objective, to be consistent with other food contact materials. In particular, a substance is acceptable if its specific migration does not exceed: 10 ppb, in case of insufficient toxicological data; 50 ppb if three negative mutagenicity tests requested by EFSA4 Guidelines are available; above 50 ppb, if supported by favorable toxicological data and/or evaluation done in accordance with the EFSA Guidelines” (Extract from EuPIA Guideline on Printing Inks applied to the non-food contact surface of food packaging materials and articles, September 2009)
EUPIA also provides guidelines on how to measure the potential level of migratables arising from printed matter. For inks applied to the non-food contact surface of packaging (i.e. the outer surface), whether that be to the primary packaging or secondary packaging (labels and sleeves) then the most likely route for migratable species from the ink contaminating the foodstuff is by what is known as set-off migration. This is where printed matter is stacked or reeled prior to it being filled with food. Thus, the ink comes into contact with what will be the food-contact surface of the package and migratable components of the ink can diffuse into this surface. When the package is then filled with foodstuff, the contaminants from the ink which have diffused into the contact-surface of the package can then leach into the food causing a potential contamination issue. This is shown in FIG. 1.
Thus, any UV-curable inkjet fluid which is applied to either the primary or secondary packaging of foodstuff should not result in contamination of that foodstuff at levels exceeding the limits detailed above.
It is predicted that UV-curable inkjet will become more prevalent in the food packaging sector, especially in the narrow web market where it will start to supplant more traditional printing methods such as UV flexo. This is the result of there being a drive to shorter print runs where the start-up costs of UV flexo and gravure become prohibitive. A particular market which is likely to be addressed by UV inkjet is the label and sleeve market.
The prior art identified describing radiation curable inkjet fluids having low migration potential includes WO2009/053348 which describes how UV-curable fluids having low migration potential can be formed by combining a specific blend of mono-, and difunctional acrylates, a polymerizable component containing both an acrylate and a vinyl ether group (specifically ‘VEEA’; 2-(2-vinyloxyethoxy)ethyl acrylate), along with curable materials having greater than 3 polymerizable groups. It states that polymeric, polymerizable and multifunctional photoinitiators (PI's) may also be used. However, this patent publication does not adequately describe how to achieve sufficiently low migratables when inks are UV-cured under air and nor does it describe how to achieve inks having sufficiently low viscosity for the newer types of inkjet print-heads.
WO2009/030658 describes a blend of photoinitiators having a low migration potential, with the proviso that the compositions contain an aromatic polymerizable tertiary amine co-initiator. However, like WO2009/053348 this patent does not describe how to achieve acceptable migratable levels from pigmented inkjet fluids when cured in air.
It is commonly held that the approach to achieving low migratable UV-curable inks and varnishes (e.g. offset, flexo, gravure, inkjet) is to use a combination of polymeric photoinitiators along with lower concentrations of other photoinitiators, in such a combination that, when cured, the polymerized fluids (inks and varnishes) have sufficiently low concentrations of unreacted monomer and free photoinitiator that migrate into foods from printed packaging at levels below regulatory requirements. In Europe, the limits for most monomers migrating into foods are less than 10 ppb (1 ppb=1 μg/Kg(food)), unless a SML applies. In the case of photoinitiators, polymeric types are assumed not to migrate due to their high molecular weight, although SMLs of 50 ppb commonly apply (as a result of there being 3 independent mutagenic test results for the material).
However, the prior art does not teach any low migration UV-curable fluid, intended for the graphic arts and relying on the use of photoinitiators to bring about the polymerization of the unsaturated monomers, that do not contain polymeric or high molecular weight photoinitiators.
A significant advantage of using photoinitiators having theoretical molecular weights of less than 500 amu (atomic mass unit—determined using a simple mathematical equation based on the atomic mass of the constituents), especially in inkjet formulations, is that it allows low viscosity photopolymerizable fluids and especially inks to be produced. Because of this, they can furthermore allow higher pigmentation levels, which would be highly beneficial in graphics (packaging) applications and yet furthermore will also allow the incorporation of monomers, oligomers and other components having relatively high viscosities into the ink to provide extra benefit (e.g. alkoxylated tri-, and higher functional acrylates, polyurethane acrylates, epoxy acrylates, inert resins, etc.).
U.S. Pat. No. 8,217,095 describes UV-curable inkjet compositions comprising a blend of photoinitiators that can contain difunctional types including bis-phosphine oxide types and difunctional hydroxyketone types. However, the ink compositions contain a significant quantity of monofunctional monomer (11% (w/w)) which would likely lead to high levels of unreacted monomer and consequently high levels of migratable monomer and thence unacceptable contamination of any foodstuff contained within a food package printed with such an ink. Furthermore, the inks of this disclosure may also contain significant amounts of monofunctional photoinitiators which will also be prone to being unreacted and therefore available to cause contamination of any foodstuff. And, most significantly, the inks of this disclosure have not been demonstrated as being suitable for the printing of food packaging. Indeed, compositions prepared according to this disclosure result in unacceptable levels of migratable components from cured ink films, which would make them unsuitable for food packaging applications.
It is a surprising finding of the present invention that UV-curable inks having acceptable migratable levels can be produced by using only low molecular weight photoinitiators without recourse to polymeric or polymerizable photoinitiators.
The advantages over the prior art, particularly for UV-curable inkjet fluids cured in air, are clear. The approach described herein allows the formulation of pigmented inkjet products which when cured can produce levels of migratable species below their SMLs where they apply or of less than 10 ppb (as a level of contamination in a simulated foodstuff) for each potentially migratable component. Furthermore, the use of only low molecular weight (yet low migration potential) photoinitiators allows the preparation of low viscosity inks with acceptable pigmentation levels for the inkjet printing of material for graphics (packaging) applications. There is a drive to lower viscosities for inkjet products for instance, the KJ4A printhead (ex. Kyocera) has been found to require UV-curable inkjet fluids having viscosities of around 6.5 mPa·s at temperatures between 40-45° C. With typical pigmentation levels of greater than 2.5% (w/w) being necessary for single pass applications, this places a great deal of restriction on how to formulate UV-curable inkjet products which, when cured under air at acceptable press speeds (greater than 20 m/min), produce cured inks with levels of migratable components falling below regulatory requirements. In particular, it is highly desirable that the level of migratable monomer falls below the 10 ppb level. This would preclude the use of any significant amount of low viscosity monofunctional monomer in the ink; an approach used in other inkjet technologies to help reduce the viscosity, including that of U.S. Pat. No. 8,217,095. Indeed, U.S. Pat. No. 8,217,095 does not describe how such low viscosity inks can be obtained, let alone inks having the required low migration potential for the printing of food packaging.
WO2009/053348A1 does not describe how low viscosity inkjet products producing acceptable migratable levels when cured in air can be produced. Indeed, when inks have been prepared according to some of the examples of this application, they have been found to be high in viscosity, to have poor cure response when cured in air and to produce unacceptable levels of migratables even when cured at very high cure doses.
Inks prepared according to the prior art (WO2009/053348) have been shown to cure poorly in air and to produce high levels of migratables from cured ink films even when cured using high doses of UV-light. Inks prepared according to U.S. Pat. No. 8,217,095 have unacceptably high viscosity and are prone to producing high and unacceptable levels of migratable monomer. Furthermore, U.S. Pat. No. 8,217,095 does not sufficiently define the predominant use of low migration potential photoinitiators. Yet furthermore, inks prepared according to the prior art examples do not have sufficiently low viscosity for use in the newer printheads, including those from Kyocera. The prior art does mention that difunctional photoinitiators have low migration potential and low impact on viscosity but does not show how to prepare effective low migration inks that cure under air, using predominantly these types of photoinitiators. Indeed, the majority of the examples described in the prior art (WO2009/053348) were cured under nitrogen so as to exclude the oxygen which would otherwise cause inhibition of the cure and lead to higher levels of migratable material.
It has been shown by the present invention that it is possible to achieve low migratable levels by using predominantly only difunctional monomers and oligomers, whereas WO2009/053348 requires that when the concentration of mono- or difunctional (acrylate) monomers exceeds 24% (w/w) of the polymerizable content then they must also contain a proportion of tri-, and/or higher functional acrylates to deliver the required performance. An issue with using higher functional acrylates of this type is that they are generally higher in viscosity than low molecular weight difunctional acrylates such as DPGDA (dipropyleneglycol diacrylate), and would therefore tend to increase the viscosity of the resultant inkjet fluid. Therefore, for the low viscosity inkjet applications now becoming more common, their use would be restricted. Inkjet inks prepared according to the present invention can achieve acceptable migratable levels, at sufficiently low viscosity, by using only difunctional monomers.
As indicated above, the present invention relates to low viscosity pigmented inkjet compositions which are able to deliver low levels of migratables when UV-cured in ambient air at commercially acceptable UV-doses, by the use of low molecular weight, non-polymeric, monofunctional, difunctional (or trifunctional) photoinitiators having molecular weights below 500 amu. The prior art does not disclose any other UV-curable ink compositions which can achieve this solely by the use of such low molecular weight photoinitiators and they invariably incorporate what are known as polymeric photoinitiators where photoinitiator moieties such as thioxanthone, benzophenone and aminobenzoate are reacted onto a polymeric backbone. These polymeric backbones are usually poly(alkylene) oxides, such as poly(ethylene glycol), poly(propylene glycol), or polycarbonate diols, such as poly(caprolactone). These polymeric photoinitiators are diffusion restricted in UV-cured inks/varnishes thereby limiting the amount of these photoinitiators that can migrate from cured films and cause contamination of sensitive packaged produce such as food and pharmaceuticals.
A further advantage of the inks of the current invention is that, compared with those described in WO2009/053348, they do not require the incorporation of tri-, and higher, functional acrylates to deliver the required low migratable levels from UV-cured films, This is a distinct advantage in being able to deliver low viscosity fluids, as the higher functional acrylates tend to have higher viscosities than difunctional types. By not requiring the use of higher functional acrylates in substantial amounts as demonstrated in the prior art, Inkjet fluids having viscosities of less than 7.5 mPa·s at temperatures of 45° C. and lower can be readily achieved. Furthermore, the comparative examples COMP-1 to COMP-4 of WO2009/053348 which are photopolymerizable varnishes comprising VEEA and DPGDA with no higher functional monomers, indicate that cured films having low levels of migratable (or uncured material) cannot be achieved. This has clearly been demonstrated otherwise in the current invention.